Smart phones are indispensable devices for modern people. Since smart phones have high values, the smart phones are usually the targets of the thieves. Conventionally, the smart phone has an anti-theft function. If the smart phone is lost and the person who picks up the smart phone tries to unlock the smart phone, the anti-theft function secures the smart phone. For example, if the identification process fails, the smart phone is automatically locked and unable to be operated. However, the conventional anti-theft function is not helpful for the user to retrieve the smart phone and warn the thief. In other words, the anti-theft function of the conventional smart phone needs to be further improved.